Power amplifiers are widely used in communication systems, for example in radio base stations and cellular phones of a cellular radio network. In such cellular radio network, power amplifiers typically amplify signals of high frequencies for providing a radio transmission signal. A consideration in the design of power amplifiers is the efficiency thereof. High efficiency is generally desirable so as to reduce the amount of power that is dissipated as heat. Moreover, in many applications, such as in a satellite or a cellular phone, the amount of power that is available may be limited due to powering by a battery, included in e.g. the satellite. An increase in efficiency of the power amplifier would allow an increase of operational time between charging of the battery.
A conventional Power Amplifier (PA), such as Class B, AB, F, has a fixed Radio Frequency (RF) load resistance and a fixed Direct Current (DC) voltage supply. The RF output current of a Class B or AB PA has a form similar to that of a pulse train comprising half wave rectified sinusoid current pulses. The DC Current, and hence DC power, is largely proportional to the RF output current amplitude. The output power, however, is proportional to the RF output current squared. An efficiency of the conventional power amplifier, i.e. output power divided by DC power, is therefore also proportional to the RF output current amplitude. The average efficiency is consequentially low when amplifying signals that on average have a low output signal amplitude, or power, compared to the maximum required output signal amplitude.
Known RF power amplifiers include both Doherty and Chireix type power amplifiers, i.e. a modified class B radio frequency amplifier. These kinds of RF PAs are generally more efficient than the conventional amplifier described above for amplitude-modulated signals with high Peak-to-Average Ratio (PAR), since they have a lower average sum of output currents from the transistors comprised in the PA. Reduced average output current means high average efficiency. In this context, the term “composite power amplifier” refers to power amplifiers which may be operated in one or more modes, such as a pure or detuned Doherty, Chireix, combined Doherty/Chireix or combined Chireix/Doherty mode, etc.
The reduced average output current is obtained by using two transistors that influence each other's output voltages and currents through a reactive output network, which is coupled to a load. By driving the constituent transistors with the right amplitudes and phases, the sum of RF output currents is reduced at all input signal levels within an operating range, except a maximum of the operating range. Also for these power amplifiers the RF voltage at one or both transistor outputs is increased.
Generally, RF power amplifier can be driven in a so called backed off operation. This means that the power amplifier is operated at a certain level, e.g. expressed as a number of decibels (dBs), under its maximum output power. Backed off operation may also refer to that an instantaneous output power is relatively low.
Wideband Doherty amplifiers are a subject of much interest, and many approaches have been attempted. For example, using a quarter wavelength transmission line with the same impedance as the load results in wideband efficiency at the transition point, as disclosed in a paper by D. Gustafsson et al., entitled “Theory and design of a novel wideband and reconfigurable high average efficiency amplifier, Proc. IMS 2012. In a standard Doherty amplifier, the transition point is at half the maximum output voltage.
In patent application WO2003/061115, filed by the present Applicant, a wideband amplifier with 100% relative bandwidth, i.e. having a 3:1 of high band edge to low band edge ratio is disclosed. The central mode of such an amplifier is a wideband Doherty mode. The disclosed wideband amplifier comprises two-stage high efficiency amplifiers with increased robustness against circuit variations and with radically increased bandwidth of high efficiency.
The wideband multistage amplifiers disclosed in patent application WO2003/061115 or PCT/SE2013/051217 have different operating modes in different frequency bands, which results in a disadvantage of complicating the input drive circuits. The central Doherty mode of the amplifier in WO2003/061115 can be up to about 60% wideband, but output signal amplitude at the transition point then varies considerably within the bandwidth.
A Doherty amplifier that has a quarter wavelength line with the same impedance as the load, for example as disclosed in the paper mentioned above by Gustafsson et al., has the disadvantage of requiring a different supply voltage to each of the two sub-amplifiers. This results in an oversized and underutilized main transistor in case the same technology is used for both sub-amplifiers. The wideband efficiency at the transition point is obtained by sacrificing both wideband transistor utilization and efficiency at maximum power, which reduces the bandwidth of high average efficiency as well as increases transistor cost.
Using an LC-resonator, for example as disclosed in a paper by M. Naseri Ali Abadi et al., entitled “An Extended Bandwidth Doherty Power Amplifier using a Novel Output Combiner”, Proc. IMS 2014, or using a resonant stub at the output node has the drawback of decreasing the full power bandwidth and efficiency bandwidth at full power.
In a paper by Piazzon et al., entitled “A method for Designing Broadband Doherty Power Amplifiers”, Progress in Electromagnetics Research, Vol. 145, pp 319-331, 2014, or in a paper by R Giofrè et al., entitled “A Distributed Matching/Combining Network Suitable to Design Doherty Power Amplifiers Covering More Than an Octave Bandwidth”, Proc. IMS 2014, another technique involving the use of a multi-section branch line coupler is disclosed. This technique has a limitation in the efficiency bandwidth both at the transition point and at full power, and also has a limitation in power bandwidth at full power